(1) Field of the Invention
The present invention principally relates to a method for the detection of irradiation treatment of foods and a kit therefor.
(2) Description of the Related Art
Conventionally, food products have been processed by ionizing radiation in order to inhibit germination; slow down the ripening; sterilize; kill insect pests; improve the restoration ability of dehydrated vegetables, the extraction efficiency of an active ingredient, flavor; etc. The irradiation treatment has advantages in that it is applicable to packaged end-products or frozen foods; mass treatment is possible; loss of flavor, taste, and nutrients can be reduced; and the treatment does not induce radioactivity in food because the radiation energy is finally converted into heat energy.
Radiation sources approved for food irradiation are γ rays of [60Co] or [137Cs], electron beams with 10 MeV or less, and X-rays with 5 MeV or less.
In 1981, with respect to the safety of irradiated foods, the Joint FAO/IAEA/WHO Expert Committee concluded that “food irradiation with a dosage not exceeding 10 kGy will not prejudice the wholesomeness of the foods”. Furthermore, in 1997, WHO concluded that foods irradiated with a dosage ranging from 10 kGy to 57 kGy are both safe to consume and nutritionally adequate”.
In the EU (European Union), the BCR (Community Bureau of Reference) programme undertook a research project to develop standardised detection methods from 1990 to 1993, and examined detection methods that had been studied up to then. Based on the research and examination results, nine standard reference methods were established in the EU by 2002 (Table 1). Note that these standard methods are adopted as General Codex Methods, which was accepted at the Codex general meetings in 2001 and 2003.
TABLE 1MethodEN numberAnalysis targetGC analysis ofEN 1784Irradiated food containing fathydrocarbonsGC/MS analysis of 2-EN 1785Irradiated food containing fatalkylcylobutanones(2-ACBs)Electron Spin ResonanceEN 1786Irradiated food containing(ESR) spectroscopyboneElectron Spin ResonanceEN 1787Irradiated food containing(ESR) spectroscopycelluloseThermoluminescence (TL)EN 1788Irradiated food from whichdetectionsilicate minerals can beisolatedElectron Spin ResonanceEN 13708Irradiated food containing(ESR) spectroscopycrystalline sugarDirect Epifluorescent FilterEN 13783Irradiated foodTechnique/Aerobic Plate(screening method)Count (DEFT/APC)DNA Comet AssayEN 13784Irradiated food(screening method)PhotostimulatedEN 13751Irradiated food containingLuminescence (PSL)silicate minerals(screening method)GC: gas chromatograph, 2-ACBs: 2-alkylcyclobutanones, GC/MS: gas chromatograph/mass spectrometry, ESR: electron spin resonance spectroscopy, TL: Thermoluminescence, DEPT/APC: direct epifluorescent filter technique/aerobic plate count, PSL: photostimulated luminescence
In addition to these analytical methods, numerous physical, chemical, and biological detection methods are known, but in most cases they have not been technically established (Table 2).
TABLE 2Method (principle)EvaluationAnalysis targetPhysical detection methodsElectrical impedance measurement methodBIrradiated potatoViscosity measurement methodCIrradiated pepperDifferential scanning calorimeter (DSC) methodAIrradiated fish, shrimp, egg whiteNear-infrared spectroscopy (NIR)AIrradiated spicesChemiluminescent methodBIrradiated frozen chicken,wheatAIrradiated shellfish, crustacean, chickenboneChemical detection methodo-tyrosine methodBIrradiated chicken, shrimp, shellfish, fish, frogleg, egg whiteMS crosslinking analysis of proteinsAIrradiated fish, shrimp, egg whiteLow molecularization (Reduced allergenicty)AIrradiated shrimp, milk proteinImmunoassay of 2-ACBsBIrradiated food containing fatDegraded nucleic acid base (Immunoassay)BIrradiated wheat shrimpCleavage of nucleic acid (pulsefield electrophoresis)BIrradiated beef liver, meat fish, shrimpBiological detection methodsInhibition of cell divisionBIrradiated onion, tubersDeletion of re-epithelialization of the woundBIrradiated potatoInhibition of germination abilityCIrradiated citrus(Half embryo test)BIrradiated apple, grains, potato, etcchromosomal aberrationAIrradiated grainsVaried floraBIrradiated strawberries, fish, shrimpLimulus testDIrradiated chicken(LAL/GNB)A: Concept is promising.B: Further interlaboratory trials are desirable.C: Crosscheck is in a preparation state or completed.D: Validity has been verified by the results of a laboratory crosscheck. (Setsuko TODOROKI, RADIOISOTOPES, 2000, 49: pp. 467-469, partially modified)DSC: differential scanning calorimeter, NIR: near infrared, LAL/GNB: Limulus amoebocyte lysate/Gram negative bacterial count.
A method using a pulsed PSL system that was carried out in Britain yielded satisfactory results among the above-mentioned prior methods. However, it later turned out that this method is strongly influenced by the preservation state of the sample, and it may result in signal fade when the foods are stored in an unsuitable manner or heat-treated. Detection methods whose reliability has been established to some extent (hydrocarbon method, 2-ACBs method, etc.) require an expensive measurement device or a skilled engineer, and thus such a method is of low practicality in view of measurement time and cost performance.
As analytical methods using an antigen-antibody reaction, the above-mentioned immunoassay detection methods that detect 2-alkylcyclobutanones (2-ACBs) or damage(s) in nucleic acid bases have been attempted, but the methods have not been accepted as an official analytical method.
As is clear from the above, the conventional detection methods have some drawbacks in that the methods are sensitive to the preservation state of the sample; an expensive device or a skilled engineer is required; it takes a long time; the cost is high; or the methods are not applicable to heat-sterilized foods.
The standards such as the Codex International Food Standards prescribe that irradiated foods are to be labeled on the package so that consumers can easily understand that the food has been treated by irradiation. Thus, consumer judgment as to whether a food has been treated by irradiation depends on the label according to the Standards. In order to popularize the irradiation treatment technique and to enable consumers to make an informed choice with respect to irradiated food, a simple detection method needs to be popularized at food handling sites which can appropriately detect irradiation treatment of foods not depending on the label. Furthermore, even when food has been appropriately irradiated, there are some cases in which such irradiated food is unfairly labeled and imported. Therefore, in addition to the labeling requirements under the Food Standard, it is necessary to actually detect irradiation treatment of foods so as to avoid importing such unfairly labeled irradiated foods.
Therefore, a method has been strongly demanded which simply and quickly detects irradiation treatment of foods while requiring no expensive device and any skilled engineer as is required in the conventional method.
The irradiation treatment is known to have an influence on various natural high-molecular weight compounds, leading to fragmentation and polymerization of the irradiated natural high-molecular weight compounds. In connection with an immunological reaction, several researches on the reduced allergenicity of irradiated allergens have been conducted. (Yang J-S., et al., Radiat. Phys. Chem., 1996, 48: 731-735 (ovomucoid, ovalbumin); Byun M-W., et al., J. Food Prot., 2000, 63: 940-944 (shrimp allergen, HSP); Lee J-W., et al., J. Food Prot. 2001, 64:272-276 (milkα-casein, β-lactoglobulin); Katial R K., et al., J. Allergy Clin. Immunol. 2002, 110: 215-219 (electron-irradiation sterilization and reduction/disappearance of allergenicity).
Moreover, protein fragmentation due to irradiation has been clarified by Kume, et al., (Kume T., et al., J. Sci. Food Agric., 1994, 65: 1-4).
An object of the invention is to provide a novel method for the detection of irradiation treatment of foods.